Expansion Joints, Bellows and Metal Hoses (Part 2)

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Metal expansion joints are primarily needed for the thermal expansion of piping. If straight pipe sections between pipe anchors expanded without added flexibility, the load on the anchors would likely be too much for the system to withstand.

If there are no anchors and the pipe heats up due to thermal expansion, the pipe will grow between the fixed anchors and bend – which could cripple, buckle or crack the pipe. This damage could lead to the buildup of an immense thermal load on anchors, causing them to fail. If expansion joints have been added between the anchors and the pipe were to heat up and expand, the joints would compress to compensate for the movement, with minimal thermal loads. Adding metal joints is therefore a possible way to handle thermal expansion of pipes.

By Gobind Khiani, M.Eng, P.Eng, Fellow – Piping/Pipelines, SME

Mitigating Potential Pipe Issues

Looking at an anchored piping system with an expansion joint, it is clear that the pipe will be pressurized prior to heating up. At this stage, the tie rods work to absorb the thrust load so that the anchors are not required to. In some cases, the tie rods will disengage from the lugs when the system starts to heat up, and the thrust load will be transferred to the anchors. If this scenario takes place, the anchors could also fail. To ensure that the system does not fail, the following solution can be implemented.

Ensure the anchors are large enough to withstand the thrust load. If the correct requirements are established, the rise in pressure and temperature will not negatively impact the system. The anchors must therefore be designed for the forces of the thrust load; the force is equal to the effective cross sectional area of the bellows, multiplied by the pressure. In addition to the thrust load, a second load – the spring load – is added to the anchors when the joint moves; this is equal to the joint spring rate multiplied by the movement. The spring load is usually considerably smaller than the thrust load.

The effective area and spring rate parameters of the joint are listed by the manufacturer in the joint’s technical manuals. These rates normally act as the input parameters required for computer piping design models.

Additional Components

Braided expansion joints: These are composed of short braided flexible hose sections that consists of a stainless steel corrugated piece/sheet metal in a stainless steel braid. These are referred to only as flexible connectors because they are not technically an expansion joint. That is to say, they are not meant for axial compression or extension because of the braid. As the braided expansion joint can withstand the thrust load because of its axial stiffness and extension, it essentially acts like a tie rod. Braided expansion joints are therefore most often used on pumps and other equipment as flexible connectors to eliminate the thrust load on the equipment nozzles, which act as anchors. Although they cannot absorb the thermal growth compression, the joints can move for lateral offset and absorb vibrations well. Flexible pipe connectors are usually either braided metal types or spherical rubber joints with tie rods.

Flexible loops: Like straight metal expansion joints, the flexible loops are made with corrugated braided metal hose but with two independent legs. They can move in all three directions, i.e. axial, lateral and angular (including axial compression and extension), exceptionally well. This ability to move, combined with the fact that the braid absorbs the thrust load, makes the loops a key alternative in situations where thrust load may be difficult for anchors to handle. The lack of thrust load combined with larger movements in all three directions also makes them key for absorbing seismic movements. The advantages of the flexible loops in thermal expansion application is the lack of thrust load and low spring load on the anchors. The reduction of space and extra insulation work, when compared to a hard pipe loop, are also significant.

Additional application details: There are special considerations to eliminate the thrust load for thermal growth applications. For example, a configuration, namely Zee band (Z), which uses a universal joint with tie rods. As described earlier, a universal expansion joint uses two bellows separated by a spool pipe section to allow long lateral movements. If the joint is placed where it will only move laterally but not axially, the added tie rods will never be forced to disengage. This can be set up for piping thermal expansion by placing the joint perpendicular to the main pipe run (the anchors will be far apart). This configuration can be quite helpful especially in larger diameter piping systems.

We can contrast the universal joint configuration design for lateral movement with another double bellows configuration by using an anchor base. Sometimes it is advantageous to obtain twice the amount of axial movement by placing a double expansion joint in the middle of a pipe run. The anchor base essentially splits the length of pipe between main anchors in half with a single joint acting on each half. This configuration can also be used with externally pressurized joints. In this case the thrust and spring loads from each bellows acts on the anchor base in opposite directions which cancels them out. Without these loads this anchor is referred to as an intermediate anchor as opposed to a main anchor. Main anchors are normally positioned at major changes in the pipe direction and must be designed to consider the thrust and spring loads from the expansion joints.

In addition to special anchor applications, pipe guides are also often used with metal expansion joints. Pipe guides allow movement in the axial direction only. This limitation prevents the possibility of buckling instability in a pressurized pipe run with an expansion joint. With no guides, it is possible that the pipe run with the joint may buckle as it expands. Buckling guides are added to stabilize a system that moves as expected. The spider type guides are the most popular and include an x-shaped spider that attaches to and moves with the pipe. These are not designed to take any loads from the main pipe beyond keeping it stable. In a horizontal pipe run it is possible to use slide guides that can take a downward load. The fixed bottom plate is most commonly Teflon lined to allow sliding and has rating for the load. The pipe is normally welded to the sliding portion in the field; it can also be made with clamps if preferred.

Picking a Hose That is Used in Canada as STAMPED

Hose and coupling manufactures, and/or distributors, assemble hose styles consisting of flexible metal hoses, industrial rubber hoses and polytetrafluoroethylene (PTFE) or comparable thermoplastic constructions. All hoses are specified based of particular application data. As is the standard and current industry practice, manufacturers and distributors of hoses have adopted the acronym STAMPED to over many years of testing and overall performance in the field.

Interestingly enough, coupling selection is often determined by the end user, however engineering teams are regularly called upon to make informed suggestions. Manufacturer’s hoses can accept any number of required coupling styles. Most often, as long as they fit weld procedures, or can fall within a crimp specification for industrial rubber or PTFE hose, most manufacturers complete the assembly in-house.

Providing solutions in relation to difficult and severe service applications and incidents, in which clients have experienced hose/coupling collapse, is one aspect of value engineering. These setbacks potentially translate into great revenue losses for the client. Additionally, providing new and efficient designs, in accordance with Canadian Registration Number (CRN) approvals, and engineered flow calculations (determining sizing and applications) allows manufacturers to provide the solutions that engineers and clients need, to ensure safe and effective operations.

References

  • Numerous Bellow Expansion Joint Manufacturers worldwide.
  • Expansion joint manufacturers association (www.EJMA.org)
  • Fluid Sealing Association (www.FSA.org)
  • Tytec LP, Edmonton, Alberta, Canada
  • Knowledge share during meetings with various industry colleagues.